The simplest and best way to make cold-leached acorn flour, which produces a superior final product compared to hot-leached acorn flour.
Gather the largest acorns possible. The type of acorns is irrelevant, even though tannin levels vary by oak species.
Cut open acorns with a large kitchen knife using the "back porch chop" method detailed in article (also includes photos).
Discard any bad acorns (moldy, worm-eaten, etc).
Place any acorn nuts that immediately come out of the shell and testa (the papery membrane over the nut) into a bowl of water to prevent oxidation. Depending on acorn species and moisture levels, you may be able to pulverize these with water in a blender immediately to start cold leaching, skipping a couple steps.
Any good acorn nuts that stick to the shells and testas should be placed on baking sheet for flash-roasting.
Preheat oven to 350°F (177°C) and place "stuck acorns" in oven for 30 minutes on middle oven rack. Then let acorns cool to room temperature before removing nuts. (They should now be easy to remove.)
Place de-shelled flash-roasted acorns in glass bowl or jar, then cover with cold water. Put in fridge for 2-3 days, dumping and replacing water at least twice per day. Once you think they've adequately softened, do a small test batch in your blender or food processor using ~1 cup of acorns and 1 cup of water. Add more water if necessary to keep the blades turning. Our blender works better than our food processor for this step! The goal is to end up with a ground acorn meal wherein the tannins can be completely removed.
Put acorn meal into glass jar(s) and back into fridge. Ideal ratios: 40% acorn meal, 40% water, 20% air/gap at the top of jar. Carefully drain out the top layer of dark, tannin-rich water at least twice per day, being careful not to let any of the white-colored sediment pour out (that's good fats and starches!) Use a fine mesh strainer or cheese cloth if necessary. After refilling with water, shake jar vigorously to make sure acorn meal at bottom of jar also gets shaken up.
Expect 7-14 days of cold-leaching to complete tannin removal process. After 5 days, start tasting a small amount of the acorn meal during water change-outs. Give it a good chew to detect any bitter tannins still inside the small bits. Even after you no longer detect bitter tannins, we’d still recommend going 1-2 more days of leaching just to be sure.
Pour off the final bit of tannin water in jar, then pour the finished acorn meal into a bowl. Stir.
If you have a food dehydrator: spread the acorn meal 1/3" - 1/2" thick on to drying racks that are covered with foil that is folded to form a lip on all sides. Dry on low heat, no more than 115°F (46°C). Make sure it’s bone dry before finishing.
If you do not have a dehydrator, air dry acorn meal as follows: thinly spread the meal on to a baking sheet covered with foil, with the foil folded up to form a lip on all sides. Dry under a fan until completely, powder-dry, stirring 1-2 times per day to provide air exposure.
Any moisture left in can cause the finished flour to go rancid or moldy during storage, so make sure it's completely dry before going to next step!
Put dried acorn meal into a food processor or multi-bladed blender and pulverize it into a flour-like consistency. For a finer-textured acorn flour, you can either:
a. Sift the flour through a mesh strainer over a large bowl, then re-grind the larger bits that stick in the strainer; or
b. (If you have a spice grinder) Store gritty acorn flour, then use your spice grinder to finely grind the acorn flour as-needed before making a recipe.
Store acorn flour in airtight jars or bags. Label containers, including adding date processed. Stored at room temp, acorn flour can last at least 1 year. In your freezer, it can last for many years.
How to make cold-leached acorn flour - best method! https://www.tyrantfarms.com/diy-how-to-make-acorn-flour-acorn-recipes/